1. Field of the Invention
This invention relates to new and improved birefringent optical wavelength multiplexers/demultiplexers. Accordingly, it is a general object of this invention to provide new and improved devices of such character.
2. Purpose of the Invention
Optical wavelength division multiplexing is a technique for combining two or more light beams with different wavelengths along a single optical path. Optical wavelength division demultiplexing involves the separation of these signals from one another at the other end of that path. Optical multiplexers and demultiplexers are often interchangeable with each other.
Optical wavelength division multiplexing and demultiplexing can be used in optical communications systems to multiply the effective information capacity (or signal bandwidth) of a single optical communication pathway. The medium may be optical fiber, free space (or air), water, etc. Individual channels can be transmitted along the path on different optical wavelength carriers, each of which propagates independently of the others.
The multiplexers and demultiplexers described herein (sometimes generically referred to as "multiplexers") provide for an arbitrarily small separation between channel wavelengths so that a required number of channels can be located within an acceptable wavelength range therefor. This feature is especially desirable in dense media, such as glass fibers or water, since such media generally have a very limited wavelength range over which the chromatic optical dispersion is near zero and the total losses are small. For example, for glass, the wavelength range might be approximately 60 nanometers, depending upon the glass. (Contemporary single-mode fibers used at about 1.3 .mu.m have a wavelength range thereabout.) Outside of this narrow range, additional channels are of little value.
The channel spacing of the multiplexed signals can be as small as desired, as will be made more apparent hereinafter. The size of the spacing is limited primarily by the stability and spectral width of the optical sources that are utilized, such as lasers.
The locations of and spacings between the channel wavelengths can be determined by the thicknesses of prisms in the multiplexers, and these are the only fabrication parameters which generally need be changed to customize the units for a specific application. The devices can be continually tuned by varying the effective thicknesses of the prisms. In the case of a two-channel system, a multiplexer and a demultiplexer can be independently tuned to exactly match the wavelengths of two lasers in the system, so that the laser wavelengths do not have to be precisely specified. Although the invention is primarily applicable to a fiber optical system, it is to be understood that its precepts can be applied to free-space and other optical communications applications, as would be apparent to those skilled in the art.
3. Prior Techniques and Disadvantages Thereof
A. Dichroic beamsplitter method: In the dichroic beamsplitter method, two light signals impinge upon a beam-splitter that is designed to transmit one wavelength and to reflect the other. At the multiplexer end, one wavelength passes through the beamsplitter, and the other wavelength is incident at such an angle that it is reflected by the beamsplitter along the same optical path as the first. Both beams are then coupled by the same optics into a system fiber.
At the demultiplexer end, where the two wavelengths are separated, one wavelength passes through a beam-splitter, while the other wavelength is reflected from the beamsplitter. The individual beams thus separated can then be individually processed. To multiplex or demultiplex more than two wavelength channels, more than one device can, in principle, be used in a tree or series configuration.
Disadvantageously, in practice, such devices have not been able to handle more than three wavelengths with reasonable performance. Because it is difficult to consistently control and reproduce the transmission/reflection characteristics over many channels, with low losses and crosstalk for all channels, it is unlikely that such multiplexers will be useful in future multi-channel applications.
B. Diffraction grating method: A grating is utilized to diffract different wavelengths of light at different angles such that multiple wavelengths are separated from one another in different directions of propagation or combined to form a single beam when incident from multiple angles. Diffraction gratings have the potential advantage that a single device is able to handle more than two wavelengths, while all other previously known types can only handle two sets of wavelengths per stage.
Disadvantageously, diffraction gratings are inherently polarization sensitive. In single mode systems, the continual polarization variations in the fiber result in significant fading problems in the receiver.
Because the angles of diffraction are a sensitive function of wavelength, and are more sensitive for smaller interchannel separations than for larger ones, any change of laser wavelength causes the focused spot of light to wander from the optimal position at the end of the fiber. In demultiplexers, with reasonably wide channel spacings available, this can be somewhat accommodated by utilizing output fibers with much larger diameters than that of the system fiber, or by using detector cells themselves to collect the light.
In grating multiplexers, however, this has not been possible, since the output fiber is the small core system fiber, and in practice, a wandering misalignment has been intolerable for the system. For single-mode systems now becoming dominant in communications, or for closer channel spacings, this problem is even worse.
Because the different wavelengths propagate at several different closely spaced angles, the fabrication of such a device is extremely difficult. The fibers must be bunched very closely together, at specific directions with respect to one another. This usually requires individual adjustments with micromanipulators, or precision fiber-aligning grooves which cannot be adjusted.
C. Holographic devices: Holographic devices are essentially diffraction gratings that are made by the optical technique of holography. Often, the hologram is designed to perform lens type operations as well as diffraction, so some external optics can be eliminated.
Disadvantageously, in addition to the problems with conventional gratings, holographic gratings usually have higher throughput losses, especially when combined with focusing functions.